The present invention relates to the development of imaged lithographic plates, and especially to such plates as used for printing.
Plates of this type have a radiation-sensitive, oleophilic polymer resin coating on a hydrophilic substrate. Imaging at ultraviolet, visible, or infrared wavelengths produces regions of differential solubility. The plates are developed to remove the more soluble regions of the coating, thereby producing a substantially planographic pattern of oleophilic and hydrophilic regions. The developed plates are then ready for mounting on a cylinder of a printing press, where the plates are subjected to fountain fluid and ink for transfer of ink to a target surface according to the pattern of oleophilic and hydrophilic regions on the plate. Historically, the processing of plates involves immersion of the plate in a sump of developer as the plate passes through a plate processor. The plate then exits the sump, and is typically subjected to a rotating brush or molleton and a nip roll set prior to being rinsed. This method of development relies entirely on chemical dissolution. In an ideal imaged plate, the relatively insoluble regions experience little dissolution over a wide range of immersion dwell time in the developer solution, whereas over the same wide range of dwell time the relatively soluble regions quickly and completely dissolve.
In practice, this ideal is not achievable, because the developer solution chemistry, temperature, and dwell time must be traded off to optimize cleanout of background while retaining the small dots of coating that provide good highlights in the printed product. Such optimization typically requires that the developer solution be strongly reactive and the dwell time be of long enough duration, to dissolve all the background, while conditioning agents in the developer solution, such as surfactants, help protect the relatively insoluble regions of the coating from reacting with the reactive ingredient of the developer solution.
Solubility differences associated with developing latitude correlate with differences in cohesion (the degree to which coating material “sticks” to itself) and adhesion (the degree to which coating material “sticks” to the substrate).